Naturalist Chris Packham has called Southern Water's failure to promptly report a sewage leak in the New Forest "an absolute disgrace".
Sewage spilled from a burst pipe into a stream near Lyndhurst which flows into the Beaulieu River in August.
The national park authority said it was not told about the incident, in a Site of Special Scientific Interest (SSSI), until a week later.
Southern Water said it "deeply regrets" the environmental damage.
The Springwatch presenter, who lives in the New Forest, said: "It takes a lot to shock me, but that this is happening in 2023 is an absolute disgrace."
"To see that murk and muck flowing into this beautiful stream, doing what is unquestionably enormous ecological damage - I'm furious."
The sewage is believed to have spilled from the burst sewer main before cascading down through the car park at Lyndhurst Golf Club and into the stream.
Southern Water did not confirm how much sewage had leaked, but it is thought to have occurred overnight on 16 August and continued for most of the following day.
The company said the damaged sewer, along with another problem at a nearby waste treatment centre that happened simultaneously, has now been repaired.
The New Forest National Park Authority criticised the utility firm for not informing it of the "serious incident" until a week later.
Paul Walton, its head of environment and rural economy, said: "We are obviously concerned at the impact on water quality in a protected Site of Special Scientific Interest in the New Forest National Park.
"In light of the information given to us by Southern Water we are seeking further clarification from them about how and why this occurred and reassurance that improvements to its protocols and monitoring are implemented and effective."
Forestry England also said it was "disappointed" not to have been immediately informed.
Forest rangers discovered the leak while responding to a gate onto the main A35 being left open.
"It's vital that any pollution incidents or damage to the habitats are reported to us promptly," it said.
The Environment Agency said it was awaiting the results of its own investigations, including tests on about 300 samples from the water.
Southern Water said it "absolutely made errors" in not notifying authorities quickly enough.
The company's head of waste water, Alex Saunders, said: "That's something we've done a deep investigation into and now we've put processes in place so that, should anything happen again, that they're informed at a much earlier date within the incident."
In 2021 Southern Water was fined a total of £90m for deliberately dumping billions of litres of raw sewage at sites including the Beaulieu River.